drm/tegra: Fix NULL vs IS_ERR() check in probe()

The iommu_paging_domain_alloc() function doesn't  return NULL pointers,
it returns error pointers.  Update the check to match.

Fixes: 45c690a ("drm/tegra: Use iommu_paging_domain_alloc()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ba31cf3a-af3d-4ff1-87a8-f05aaf8c780b@stanley.mountain

Author: Dan Carpenter

drivers/gpu/drm/tegra/drm.c

@@ -1153,8 +1153,8 @@ static int host1x_drm_probe(struct host1x_device *dev)
 
 	if (host1x_drm_wants_iommu(dev) && device_iommu_mapped(dma_dev)) {
 		tegra->domain = iommu_paging_domain_alloc(dma_dev);
-		if (!tegra->domain) {
-			err = -ENOMEM;
+		if (IS_ERR(tegra->domain)) {
+			err = PTR_ERR(tegra->domain);
 			goto free;
 		}